Effect of Ligand Structure on the CuII–R OMRP Dormant Species and Its Consequences for Catalytic Radical Termination in ATRP

• Published in: Macromolecules Vol. 49; no. 20; pp. 7749 - 7757
• Main Authors: Ribelli, Thomas G, Wahidur Rahaman, S. M, Daran, Jean-Claude, Krys, Pawel, Matyjaszewski, Krzysztof, Poli, Rinaldo
• Format: Journal Article
• Language: English
• Published: American Chemical Society 25.10.2016
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/acs.macromol.6b01334

The kinetics and mechanism of catalytic radical termination (CRT) of n-butyl acrylate (BA) in MeCN in the presence of Cu complexes with tridentate and tetradentate ligands was investigated both theoretically and experimentally. The tetradentate TPMA, TPMA*1, TPMA*2, TPMA*3, and the newly synthesized tridentate N-propyl-N,N-bis­(4-methoxy-3,5-dimethylpyrid-2-ylmethyl)­amine (BPMA*Pr) as well as tridentate BPMAMe were used as ligands. L/CuIIX2 (X = Cl or OTf) complexes were characterized by cyclic voltammetry (CV), UV–vis–NIR, and X-ray diffraction. Polymerization of BA initiated by azobis­(iso­butyronitrile) (AIBN) in MeCN in the presence of a L/CuI complex showed higher rates of CRT for more reducing L/CuI complexes. The ligand denticity (tri- vs tetradentate) had a minor effect on the relative polymerization kinetics but affected the molecular weights in a way specific for ligand denticity. Quantification of the apparent CRT rate coefficients, k CRT app, showed larger values for more reducing L/CuI complexes, which correlated with the L/CuII–R (R = CH­(CH3)­(COOCH3)) bond strength, according to DFT calculations. The bond strength is mostly affected by the complex reducing power and to a lesser degree by the ligand denticity. Analysis of kinetics and molecular weights for different systems indicates that depending on the ligand nature, the rate-determining step of CRT may be either the radical addition to L/CuI to form the L/CuII–R species or the reaction of the latter species with a second radical.